US 20050157777 A1 Abstract A simplified spread spectrum demodulator uses a frequency detector to demodulate a modulated spread spectrum signal to obtain successive chip values. A correlation unit correlates the successive chip values with fixed sequences of correlation coefficients to generate correlation values. A decision circuit selects one of the correlation values to decide what symbol the spread spectrum signal represents. The correlation coefficients are obtained by applying the same modulation method as used to modulate the spread spectrum signal, and then the same demodulation method as used by the frequency detector, to the sequences of chips representing different symbols. Since synchronous detection is not employed, no carrier recovery circuit is needed.
Claims(12) 1. A demodulator receiving a spread spectrum signal in which different data symbols are represented by predetermined sequences of chips, the spread spectrum signal having been modulated by a predetermined modulation method, the demodulator comprising:
a frequency detector using a frequency detection method to generate a frequency-detected signal representing successive chip values; a correlation unit for correlating the frequency-detected signal with predetermined sequences of correlation coefficients to generate a plurality of correlation values, the predetermined sequences of correlation coefficients being obtained by modulating the predetermined sequences of chips by said modulation method and demodulating the modulated predetermined sequences of chips by said frequency detection method; and a decision circuit for selecting one of the correlation values, thereby deciding which one of the data symbols the spread spectrum signal represents. 2. The demodulator of 3. The demodulator of 4. The demodulator of 5. The demodulator of 6. The demodulator of 7. The demodulator of 8. The demodulator of 9. The demodulator of the different data symbols include a number of pairs of data symbols, the two sequences of chips representing each pair of data symbols having a fixed mutual relationship; the correlation unit includes a number of correlators equal to the number of said pairs of data symbols, and generates correlation values for one data symbol in each of the pairs of data symbols; and the decision circuit performs an operation on the correlation values generated by the correlation unit to generate correlation values for another data symbol in each of the pairs of data symbols. 10. The demodulator of 11. The demodulator of 12. The demodulator of Description 1. Field of the Invention The present invention relates to a demodulator suitable for use in, for example, a direct-sequence spread spectrum communication system employing offset quadrature phase-shift keying (OQPSK) modulation. 2. Description of the Related Art Conventional OQPSK demodulators have been described in various publications of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, including IEICE SB-3-5 (1988, p. 1-564), IEICE B-150 (1991, p. 2-150), IEICE B-200 (1992, p. 2-200), and IEICE SAT 92-2 (1992, pp. 3-8). These demodulators generally employ synchronous detectors that compare the phase of the received OQPSK modulated signal with the phase of a synchronized reference carrier signal. Various synchronous detection methods are employed, but all require at least a carrier recovery circuit to generate the reference carrier signal from the received OQPSK signal, a clock recovery circuit, a bandpass filter (BPF), and a low-pass filter (LPF). These circuits take up considerable space, especially the carrier recovery circuit, which typically includes a phase-locked loop or a reverse modulator. An OQPSK demodulator employing synchronous detection is unavoidably large and complex. There is a need for a smaller and simpler type of OQPSK demodulator. An object of the present invention is to provide a simplified demodulator for a spread-spectrum signal. The invented demodulator receives a spread spectrum signal, modulated by a predetermined modulation method, in which different data symbols are represented by predetermined sequences of chips. A frequency detector detects the received signal to generate a frequency-detected signal representing successive chip values. A correlation unit correlates the frequency-detected signal with predetermined sequences of correlation coefficients to generate a plurality of correlation values. A decision circuit selects one of the correlation values, thereby deciding which one of the data symbols the spread spectrum signal represents. The predetermined sequences of correlation coefficients are obtained by modulating the predetermined sequences of chips representing the symbol values by the same modulation method as used to modulate the spread spectrum signal, and demodulating the resulting modulated sequences of chips by the same detection method as used by the frequency detector. The modulation method may be a phase modulation method such as OQPSK. The frequency detection method may be a method of the type employed in frequency-shift keying demodulation. This type of frequency detection is inherently simpler than synchronous detection. In particular, the invented demodulator does not require a carrier recovery circuit. In the attached drawings: Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. The embodiments assume a direct-sequence spread spectrum communication system satisfying the following conditions (a) to (d). (a) OQPSK modulation is used. (b) Sixteen data symbols are coded as pseudo-random sequences of chip values with low mutual correlation (the sequences are substantially orthogonal). (c) Each pseudo-random sequence includes thirty-two (32) chips. (d) The pulse shape is defined as in The two sequences in In the direct-sequence spread spectrum communication system assumed in the following embodiments, 4-bit data symbols are spread into 32-chip sequences for transmission to the receiver. In ordinary direct-sequence spread spectrum communication, the receiver uses the same 32-chip sequences are used to despread the received signal and recover the data symbols, but in the embodiments described below, different chip sequences are used for despreading. Referring to The frequency detector The modulated signal MS The timing recovery unit Operating at this optimal timing TM Given the frequency-detected signal DT The correlation unit The correlators CR For example, the 32-chip sequence of data symbol ‘0’ in The combined phase of the modulated signal shifts among points A, B, C, and D in the phase plane shown in The first chip c All thirty-two chips representing data symbol ‘0’ are shaped into pulses according to condition (d) and OQPSK modulation is performed. The phase of the modulated signal shifts among points A to D in the phase plane in As this example shows, after OQPSK modulation and FSK demodulation, the thirty-two chip values of a data symbol in Correlation coefficients for the other fifteen data symbols (‘1’ to ‘15’) are obtained in the same way. The resulting coefficient sequences are shown in The correlators CR Aside from using different correlation coefficients, all sixteen correlators CR Referring again to The data converter Next, the operation of the first embodiment will be described. In this embodiment, a transmitter (not shown) performs the type of direct-sequence spread spectrum modulation defined by conditions (a) to (d) and transmits the resulting modulated signal MS The demodulator The timing recovery unit The correlators CR The correlation values output by the correlators CR The data converter In the type of direct-sequence spread spectrum communication defined by the conditions (a) to (d) given above, this embodiment reduces the overall circuit size of the demodulator by using a frequency detector or FSK demodulator instead of a synchronous detector that requires carrier recovery. A second embodiment will be described below, focusing on the differences from the first embodiment. The demodulator in the second embodiment takes advantage of the paired relationship among the correlation coefficients shown in Referring to The frequency detector The correlation values s Each of the eight subtractors RD The sixteen 31-chip sequences of correlation coefficients in The correlation values s Similarly, the correlation coefficients corresponding to data symbols ‘1’ and ‘9’, ‘2’ and ‘10’, ‘3’ and ‘11’, . . . , and ‘7’ and ‘15’ form complementary pairs, the difference obtained by subtractor RD Apart from the method of calculating correlation values s Although it depends on the specific circuit implementation, since a subtractor (such as RD The chip sequences in In the second embodiment, all data (chips) in the paired sequences of correlation coefficients are in a complementary relationship with one another, but any other predictable relationship can be used in a generally similar way. For example, if two sequences of correlation coefficients have identical I-phase values and complementary Q-phase values, the I-phase and Q-phase values can be correlated separately, and the sum and difference of the two results can be manipulated to obtain correlations with two different symbols. The chip sequences in the first and second embodiments are shaped into sinewave pulse sequences, but the invention can be practiced without shaping the chips into sinewave pulses. Although the first and second embodiments assume direct-sequence spread spectrum communication using OQPSK modulation, the invention can be practiced in any communication system in which the data symbols have fixed chip sequences. For example, the invention can be practiced with the complementary code keying (CCK) system specified in standard 802.11b of the Institute of Electrical and Electronics Engineers (IEEE) for use with wireless local area networks. The invention can be practiced in either hardware or software, or a combination of hardware and software. Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims. Referenced by
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